An anti-IgE monoclonal antibody that binds to IgE on CD23 but not on high-affinity IgE.Fc receptors

Abstract

A new monoclonal antibody (mAb), specific for human IgE, the central mediator of immediate-type hypersensitivity reactions, has been shown to possess a unique set of binding specificities. The mAb, 8D6, binds to a conformational epitope on the CH3 domain of human e immunoglobulin and can compete with omalizumab for binding to IgE. Like omalizumab, it does not bind to IgE bound by the high-affinity IgE.Fc receptor (FcɛRI) on basophils and mast cells. It also does not cause activation and degranulation of IgE-pulsed, human FcɛRI-expressing rat basophilic leukemic cells (RBL SX-38). The mAb can inhibit IgE binding to recombinant α chain of human FcɛRI in ELISA and to human FcɛRI-expressing RBL SX38 cells in fluorescence flow cytometric analysis. However, unlike omalizumab, 8D6 can bind to IgE already bound by the low-affinity IgE.Fc receptors (FcɛRII, or CD23), as revealed in ELISA with recombinant CD23 and in flow cytometric analysis with human B cells. Since earlier investigators have shown that anti-CD23 mAbs can inhibit the synthesis of IgE in lymphocyte culture in vitro and can down-regulate IgE production in treated patients, 8D6 may offer pharmacological mechanisms in addition to those mediated by omalizumab, for controlling IgE in patients with allergic diseases.

Introduction

Allergic diseases, such as allergic asthma, allergic rhinitis, and food allergy, are very prevalent among populations living a Westernized lifestyle (Gold and Wright, 2005, Isolauri et al., 2004). Numerous clinical studies of the humanized anti-IgE antibody, omalizumab, in many allergic diseases have shown that omalizumab is efficacious and safe in treating patients with those allergic diseases (Lanier et al., 2003, Milgrom et al., 1999, Rafi et al., 2010). These results have not only confirmed that IgE is a central mediator in the pathogenesis of many allergic diseases but have also validated IgE as a good molecular target for therapeutic intervention (Holgate et al. 2005). The widely recognized pharmacologic mechanisms of omalizumab are its ability to neutralize free IgE and to inhibit IgE binding to FcɛRI on basophils and mast cells (Chang, 2000, Chang et al., 2007). Moreover, the depletion of free IgE in the blood and in interstitial spaces causes a down-regulation of FcɛRI on basophils and mast cells over time (Beck et al., 2004, Chang and Shiung, 2006). While the ability of the antibodies to bind to membrane-bound IgE (mIgE) (Davis et al. 1993), which forms part of the B cell receptor complex (Niiro and Clark 2002), and to cause the lysis of IgE-expressing B cells and hence inhibit IgE production, was considered in the original design of the anti-IgE therapeutic antibodies, this property has not been confirmed for omalizumab in patients.

Therapeutic approaches aiming at suppressing IgE production, such as anti-interleukin 4 administration (Hart et al. 2002), TH2/TH1 modulation (Stokes and Casale 2004), and anti-CD23 treatments (Bonnefoy et al., 1990, Poole et al., 2005), have drawn much interest among researchers attempting to develop treatments for IgE-mediated allergic diseases. CD23 expressed on the surface of B cells, which binds to IgE with an association constant Ka of about 10⁸–10⁹ M⁻¹ (Gould and Sutton 2008), is known to be involved in the regulation of IgE production (Conrad et al. 2007). A mature CD23 molecule is formed by three 45-kDa type II transmembrane subunit proteins that belong to the C-type lectin family (Conrad 1990). Each subunit contains an α-helical stalk region between the extracellular lectin domain and the transmembrane domain, which bundles to form coiled-coil trimers (Beavil et al., 1992, Hibbert et al., 2005). Membrane-bound CD23 is cleaved by a metalloproteinase, ADAM10, releasing the secreted form of CD23 (Lemieux et al. 2007). It has been shown in mice in vivo that when the lectin domain of membrane-bound CD23 is bound by IgE or by an anti-CD23 antibody, CD23 becomes more stable and is less susceptible to the cleavage by ADAM 10, which signals the B cell to reduce IgE synthesis (Conrad et al., 2007, Ford et al., 2006). Several chimeric anti-CD23 mAbs were shown to cross-link CD23 and inhibit the transcription of ɛ gene and the production of IgE by human peripheral B cells in culture (Yabuuchi et al. 2002). An anti-CD23 mAb, lumiliximab, was shown to induce apoptosis by cross-linking CD23 on B cell lines or on chronic lymphocytic leukemia cells (Pathan et al. 2008). Furthermore, lumiliximab was able to reduce blood IgE levels in allergic patients (Rosenwasser et al. 2003) and is potentially applicable for treating patients with chronic lymphocytic leukemia (Byrd et al. 2007).

In this study, we have developed an anti-IgE mAb, 8D6, which resembles omalizumab in various aspects, including being able to bind to the same region on the CH3 domain of ɛ chain, not being able to bind to IgE on FcɛRI, and not being able to induce the activation of basophils. However, mAb 8D6, unlike omalizumab, has the additional attribute of being able to bind to IgE that is already bound by CD23 and hence has the potential to down-regulate IgE synthesis.